Fracture healing stages

Light Bulb Sign: A Surgeon’s Guide to Diagnosing Posterior Shoulder Dislocation

The “light bulb sign” is a classic radiographic clue to posterior shoulder dislocation—yet it’s frequently misinterpreted, leading to missed diagnoses or unnecessary interventions. This guide explains the anatomy, mechanism, imaging pitfalls, and key differentials every clinician must know.

With only 2–4% of shoulder dislocations being posterior, a high index of suspicion and precise radiographic interpretation are essential to avoid long-term complications like avascular necrosis, chronic instability, or osteoarthritis.

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Anatomy & Mechanism: Why the “Light Bulb” Appears

In a normal shoulder, the humeral head is slightly asymmetric on AP view—larger medially where it articulates with the glenoid. During posterior dislocation, powerful internal rotators (pectoralis major, latissimus dorsi, teres major) force the arm into internal rotation, rotating the humeral head so its contour appears round and symmetric—resembling a light bulb.

Visualizing the Light Bulb Sign (Text-Based Diagram)

Below is a simplified representation of the humeral head contour on AP radiograph:

NORMAL vs. POSTERIOR DISLOCATION (AP View)

NORMAL SHOULDER:        POSTERIOR DISLOCATION:
                         (Light Bulb Sign)
     _______                 _______
    /       \               /       \
   /  Asym-  \             /  Round  \
  |   metric |           |  Symmetric |
   \  Head   /             \  "Bulb"  /
    \_______/               \_______/

Medial side larger      Appears round due to
                        internal rotation
      

Figure 1: Text-based illustration of humeral head contour. In posterior dislocation, internal rotation creates a symmetric, rounded "light bulb" appearance on AP X-ray.

Radiographic Diagnosis: Beyond the Light Bulb Sign

While the light bulb sign is suggestive, it is not diagnostic alone. Always correlate with clinical findings and additional views.

Essential Radiographic Signs

Sign	Description	Significance
Light Bulb Sign	Round, symmetric humeral head on AP view	Suggestive but not definitive
Rim Sign	Glenohumeral joint space >6 mm	Indicates dislocation
Trough Line Sign	Vertical sclerotic line on medial humeral head	Reverse Hill-Sachs lesion (impaction fracture)
Scapular Y View	Humeral head anterior to “Y” formed by scapula	Gold standard for confirmation

⚠️ Critical Pitfall: The light bulb sign can appear in non-dislocated shoulders** with internal rotation (e.g., pain, guarding). Always confirm with axillary or scapular Y view**.

Clinical Red Flags for Posterior Dislocation

Suspect posterior dislocation in patients with:

• History of seizure, electrocution, or electroconvulsive therapy
• Anterior shoulder pain** with inability to externally rotate
• Arm held in internal rotation** with adduction
• Neurological deficit** (axillary nerve)

Differential Diagnosis: When It’s NOT a Dislocation

As highlighted in the Resus.blog case, the light bulb sign may occur without dislocation due to:

• Voluntary internal rotation (e.g., pain, malingering)
• Rotator cuff tear with humeral head subluxation
• Severe osteoarthritis with joint remodeling

Always obtain an axillary view** if clinical suspicion doesn’t match imaging.

Management & Complications

Once confirmed, posterior dislocations are reduced under sedation using traction-countertraction** or external rotation techniques**. Delayed diagnosis (>3 weeks) often requires open reduction.

Potential complications:

• Reverse Hill-Sachs lesion (impaction fracture)
• Avascular necrosis of humeral head
• Recurrent instability
• Post-traumatic osteoarthritis

Frequently Asked Questions (FAQs)

Q1: Can the light bulb sign appear in anterior dislocation?
A: No. Anterior dislocations show the humeral head below the coracoid** or in the axilla**—never as a symmetric light bulb.

Q2: What’s the best X-ray view to confirm posterior dislocation?
A: The axillary lateral view** is gold standard. If not possible, use the scapular Y view**.

Q3: Why are posterior dislocations often missed?
A: Subtle AP findings, low clinical suspicion, and lack of trauma history (e.g., post-seizure).

Q4: Is surgery always needed?
A: No. Acute, reducible dislocations are managed non-operatively**. Surgery is reserved for chronic, locked, or fracture-dislocations.

Conclusion

The light bulb sign is a valuable radiographic clue—but never a standalone diagnosis. Combine it with clinical context, additional views, and associated signs** to avoid misdiagnosis. In the emergency setting, when in doubt, get an axillary view**.

At BoneFractures.org, we emphasize precision in orthopedic diagnosis—because every missed dislocation risks a patient’s long-term function.

Medical Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult a qualified orthopedic surgeon for diagnosis and treatment.

References

1. Bell DJ. Light bulb sign (posterior shoulder dislocation). Radiopaedia.org. Updated May 15, 2021.
2. Kas P. The ‘Lightbulb Sign’ doesn’t always mean a posterior dislocation. Resus.blog. Orthopaedics.
3. Haas N. 'Light Bulb Sign' in Posterior Shoulder Dislocation. American Academy of Emergency Medicine. 2014.
4. Daya M, Nakamura Y. Shoulder girdle fractures and dislocations. Emergency Medicine Practice. 2007;9(10):1-28.
5. Rockwood CA, Matsen FA. The Shoulder. 5th ed. Saunders; 2014.

— Written by Dr. Mohamed Attia, Orthopedic Surgeon
Published on BoneFractures.org – Evidence-based fracture and joint care for patients and professionals.